I've tried this cave in Krissz' BD remake and succeeded to blast both walls.
With good timing it is possible to pass a firefly right above it, from left to right, without dying.
I haven't tried on the C64 version yet, but I would expect it is also possible in the original BD engine._________________---- Boulder Dash Fansite ----

There are a lot of caves out there with boulders placed like that. It is not that hard to do. You have to wait under the boulder while a firefly is on its way and move to the right in a correct moment. There are two moments which will work. Moving inbetween, and you will fail. Moving to early or too late, you would fail too, obviously.

Boulder Dash clones with inaccurate physics most likely would make this impossible to do. GDash should be very accurate with this, though.